Post Doctoral Research
Background
Soils store a vast amount of carbon, particularly in boreal ecosystems, and are sensitive to environmental change. Current model predictions of the impacts of climate change upon boreal forest soils are hindered by the lack of knowledge about rates and patterns of carbon allocation into and out of the various soil components. While microbial respiration may be controlled mainly by temperature, the crucial additional influence of plant allocation of exudates to mycorrhizae and microbes means that temperature is not necessarily a particularly accurate predictor of soil respiration. In this context, isotopic labelling of intact ecosystems offers a means to trace, with high temporal resolution, linkages between the atmosphere, plants, and soil.
Pilot Study
In 2006, a 50 m2 patch of boreal Pinus Sylvestris forest was enclosed within a transparent 200 m3 chamber and pulse-labelled with 13CO2. A relatively short period of 13CO2 exposure (93 minutes) was sufficient to label key components of the ecosystem. Tracer abundance peaked after 1 hour in needles, 24 hours in phloem carbohydrates at 0.3 m height, after 2-4 days in soil microbes and CO2 emerging from soil, and after 4-7 days in ectomycorrhizal roots. There was substantial variability among, otherwise very similar, mycorrhizal roots in terms of the degree of 13C labelling. Simultaneous labelling of the soil with 15NH4 indicated that the ectomycorrhizal roots were the strongest sinks not only of photosynthate but also of soil nitrogen. Together, these results demonstrate that in this ecosystem there is a close temporal coupling (several days) between canopy photosynthesis and a considerable portion of soil activity.
Current Work
The pilot study demonstrated that 13CO2 works well as a large-scale ecosystem tracer, and provided tantalizing insights into the rates and patterns of carbon allocation to some key plant and soil components. The key aim of the current project, therefore, is to upscale these measurements to include:
1) More plot replication: we have now labelled 8 50m2 plots. These plots consist of four pairs that each have similar vegetation and soil types. One plot of each pair has been fertilized with nitrogen, whilst the other has remained un-modified. An unlabelled plot adjacent to each pair provides measurements of 13CO2 natural abundance in plant and soil components. In addition, the pairs were labelled at different times (early and late in the growing season) and have different vegetation types (high and low tree stem density). In all, this experimental design should allow a more rigorous assessment of temporal and spatial variation in carbon allocation within this forest.
2) Measurement of more components: in addition to the components recorded in the pilot study, we have also now measured 13C labelling in the following components:
i) Plant material (tree needles of different ages, stem and roots, understory vegetation and mushrooms).
ii) Soil animals.
iii) Plant respiration (CO2 produced from branches and stems).
3) Other measurements: Collaborators from York brought their online mass spectrometer to make half-hourly measurements of 13C labelling in CO2 emerging from soil, whilst collaborators from Umeå Plant Science Centre and Vienna will be examining 13C labelling in different carbohydrates within plant material. Together these measurements should provide an extraordinarily detailed glimpse into where and when recently assimilated carbon moves through the ecosystem. This information should help current models to construct a more realistic representation of the linkages between plants and soil, to more accurately predict ecosystem responses to climate change
